Abstract
Abstract. Alluvial fans are often present at the outlet of small drainage basins in alpine valleys; their formation is due to sediment transport associated with flash floods and debris flows. Alluvial fans are preferred sites for human settlements and are frequently crossed by transport routes. In order to reduce the risk for economic activities located on or near the fan and prevent loss of lives due to floods and debris flows, torrent control works have been extensively carried out on many alpine alluvial fans. Hazard management on alluvial fans in alpine regions is dependent upon reliable procedures to evaluate variations in the frequency and severity of hydrogeomorphic processes and the long-term performance of the torrent training works. An integrated approach to the analysis of hydrogeomorphic processes and their interactions with torrent control works has been applied to a large alluvial fan in the southern Carnic Alps (northeastern Italy). Study methods encompass field observations, interpretation of aerial photographs, analysis of historical documents, and numerical modelling of debris flows. The overall performance of control works implemented in the early decades of 20th century was satisfactory, and a reduction of hazardous events was recognised from features observed in the field and in aerial photographs, as well as from the analysis of historical records. The 2-D simulation of debris flows confirms these findings, indicating that debris flow deposition would not affect urban areas or main roads, even in the case of a high-magnitude event. Present issues in the management of the studied alluvial fan are representative of situations frequently found in the European Alps and deal with the need for maintenance of the control structures and the pressures for land use changes aimed at the economic exploitation of the fan surface.
Highlights
Alluvial fans at the outlet of small, high-gradient drainage basins are a common feature of alpine valleys
Minor events have not been found in the available documentation, meaning that the actual frequency of hydrogeomorphic processes on the Rivoli Bianchi alluvial fan is likely higher than that arising from Table 1
This study has demonstrated the suitability of a comprehensive approach, based on analysis of historical documents, aerial photo interpretation, field observations and numerical modelling of debris flows, for enabling recognition of contemporary hydrogeomorphic processes and evaluation of the long-term effect of torrent control works on alluvial fans
Summary
Alluvial fans at the outlet of small, high-gradient drainage basins are a common feature of alpine valleys. Debris flows commonly dominate the formation and development of these alluvial fans, bedload and hyperconcentrated flows contribute to the transfer of sediment from the drainage basin to the alluvial fan and its distribution on the fan surface. These flow processes result in major risk when they encroach settlements and transport routes, which are often present on the alluvial fans of the European Alps. To cope with flow processes and to reduce their hazard, torrent control works have been built on many alpine alluvial fans. The study of contemporary hydrogeomorphic processes must take into account the interactions between natural phenomena and the control works implemented to reduce the risks on human activities.
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